Photoplethysmography (PPG) based heart rate detection works by detecting reflected light from blood vessels as the blood vessels dilate and contract in sympathy with changing blood pressure associated with the heartbeat. The light is generated by a pulsed Light Emitting Diode (LED) which is placed against the skin (often a wrist) and detected by a photodiode also placed against the skin in near vicinity to the LED. Since the LED has a wide transmission angle and the emitted light is subject to scattering within the body, light reflects to the photodiode from extraneous sources such as bones as well as from the blood vessels. The signal component obtained from the light reflected from extraneous sources is commonly referred to as the DC component of the received signal.
The undesired DC reflected component received is significantly greater than the signal from the blood vessel (e.g., the DC reflected component may be over 80 dB greater than the signal of interest which may typically be just 400 pA). The undesired DC component presents a number of issues. For example, amplifying the input signal to provide sufficient gain to the desired signal to detect it may lead to saturation in the amplifier stages of the PPG device.
PPG devices may wobble during use. As such, variable motion artifacts are introduced into the received photo-currents making tracking of the desired signal from the undesired signal difficult. One way to track the signal of interest from other undesirable received signals is to use a low gain, high bandwidth amplifier chain to avoid clipping (e.g., signal collapsing to ground), followed by an oversampling Analog-to-Digital Converter (ADC) (e.g., 22-bit ADC). This approach is brute-force and sub-optimal because it requires extra averaging, power burning, and is slow.